The present invention relates to a mechanical safety device for a lift in which a carriage for supporting a load is hoisted up and down a vertical hollow mast by a cable or a chain therein, and more particularly relates to a mechanical safety device designed to prevent automatically the carriage from falling down, in the event of breakage of the cable or the chain which supports the carriage.
Many lifts have been proposed and practiced in which a carriage is moved up and down a mast. For example, two-masted lifts in which two pillars are provided, with a carriage moving up and down each of the masts, each of the carriages having two arms which extend inwards, and which support a vehicle from its opposite sides so as to lift it up when the carriages are raised, are widely known. Further, it is well-known to raise at least one of the carriages up its mast by a cable attached to the carriage. Also it is well-known to provide two cables of this kind running together in parallel for the sake of safety.
The dangers inherent in these lifts are also well-known. If the supporting cable or cables should break, a very serious accident is likely. For this reason various safety devices have been proposed for lifts. As a type of safety device is known a claw which, if the support cable breaks, engages in one of a number of vertical slots cut in the support pillar. Alternatively, the slots may be provided as in a rack which is attached to the pillar. However, with these conventional types of safety devices, there is the disadvantage that when the cable breaks the claw may be midway between one slot and the next, and therefore the carriage and load will inevitably fall a certain distance before the claw engages in a lower slot. Therefore a considerable shock is caused when the claw engages, and this may either break part of the safety mechanism or, perhaps, may dislodge the load from off the carriage, and cause a further accident.
In order to remove this defect, another safety device has been proposed which is attached to the carriage and which, when the cable which supports the carriage becomes slack, locks the carriage immediately to a smooth vertical rod, without allowing it to descend any substantial distance. Further, by its construction, if the carriage is supported by two cables in parallel, this device will only operate when both the cables become slack, and therefore the carriage is otherwise unsupported.
Further, a cable break detector has also been proposed, which is attached to the mast and sounds an alarm when the cable breaks so that people who were working in a position to be endangered by the falling of the load supported by the lift should be able to get clear of it.
However, such conventional safety devices, when no electric power is supplied due to a carelessness, or an electricity failure or a trouble of the electric device, may not work any more, which means the lack of reliability and stability. In addition, most of conventional safety devices are operated manually and are high cost.